The invention relates to a former-press section of a paper machine in which the former section comprises a lower wet wire and an upper wet wire or an equivalent transfer wire, and in which the press section comprises one or more dewatering press nips, and between which former section and press section there is a pre-press zone which removes water from a web and through which the web is passed as a closed draw from the former section to the press section.
The invention also relates to a method of transferring a paper web from a former section of a paper machine to a press section thereof, which former section is provided with a lower wet wire and with an upper wet wire or with an equivalent transfer wire, and which press section is provided with one or more dewatering press nips, and between which former section and press section water is removed from the web in a pre-press zone through which the web is passed as a closed draw from the former section to the press section.
Increased speeds of paper and board machines provide new problems to be solved, which problems are mostly related to the rumnnability of the machine. Currently, speeds of up to from about 1600 to about 1700 meters per minute are employed in paper machines. At these speeds, the so-called closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced centre roll for the most part still operate satisfactorily. The applicant""s Sym-Press II(trademark) and Sym-Press O(trademark) press sections may be mentioned as examples of these press sections.
Dewatering taking place by pressing is more advantageous than dewatering by evaporation from the point of view of energy economy. For this reason, attempts should be made to remove a maximal amount of water from the web by pressing, in order that the proportion of water to be removed by evaporation could be made as low as possible. Increased speeds of paper and board machines, however, provide new, so far unsolved problems expressly for dewatering taking place by pressing because the press impulse cannot be increased sufficiently by the prior art means, above all because at high speeds the nip times remain insufficiently short and, on the other hand, the peak pressure of compression cannot be increased beyond a certain limit without destroying the structure of the web. In addition to the drawbacks mentioned above, the efficiency of the prior art suction rolls is lowered significantly at particularly high web speeds, because the suction does not have sufficient time to act upon the web in the intended manner through the relatively long perforations in the mantle of the suction roll and through non-porous and thick pick-up press felt. This both limits the efficiency of dewatering and makes the transfer of the web more difficult with the help of a vacuum in the suction roll.
With increasing speeds of paper machines, the problems of runnability of a paper machine are also manifested with higher emphasis, because a web with a high water content and low strength does not endure especially the dynamic forces produced by high web speeds and changes in the direction of the web, but rather web breaks and other operational malfunctions arise, causing downtime periods. In modern printing paper machines, the cost of downtime is today about FIM 50,000 per hour.
In the prior art press sections, the web is generally passed from the forming wire into the first press nip on a pick-up felt, which also operates as a press fabric that receives water in the first press nip, which is either a roll nip or an extended nip. In the first press nip, a relatively high compression pressure is employed and large quantities of water are dealt with, and it is one of the drawbacks arising from this that the outer face of the press felt tends to be contaminated and its porous fabric structure tends to be partially blocked. Attempts are made to prevent this by means of efficient felt conditioning devices, which are, however, quite expensive, spacious components which consume an abundance of energy.
Recently, even speeds as high as about 40 meters per second=2400 meters per minute have been contemplated as speeds of printing-paper machines. Applications at speeds as high as this, in particular in wide machines, provide ever more difficult problems to be solved, of which problems the most important ones are runnability and adequate dewatering capacity of the machine at a high web speed. Similarly, in board machines (basis weight of the web greater than 100 grams per square meter) attempts are made to increase the present web speeds (8-15 meters per second) to the level of 15-25 meters per second.
Important drawbacks of the press felts used in the prior art press sections include the effect of rewetting the web and the tendency of contamination, because, in particular when said press felts run through a high-pressure nip or nips, particles of contaminants tend to be affixed and to adhere to the press fabrics, wherefore the operation of the press fabrics is disturbed and their cleaning requires efficient conditioning devices, which consume a considerable amount of energy.
Moreover, in high-pressure press nips, the prior art porous press felts are subjected to intensive wear and strain, so that the felts must be replaced rather frequently, which increases the costs to a considerable extent.
With respect to the prior art related the invention, reference is made to U.S. Pat. Nos. 4,197,158, 4,879,001, 5,308,450 and 5,736,011. The last-mentioned U.S. Pat. No. 5,736,011 discloses a wet end of a paper machine which comprises a headbox of special construction and a twin-wire former on whose upper wire a web is passed as a closed draw to a pick-up point. The US patent does not disclose a pre-press nip nor any pre-pressing stage integral to the present invention. A further difference is that the present invention does not have the special limitations and structural requirements described in the above-mentioned US patent in connection with the headbox. The geometry of the twin-wire former and the transfer of the web from the former to a press section disclosed in the US patent are also substantially different from those of the present invention.
With respect to the prior art most closely related to the invention, reference is made to the applicant""s FI patent 98843 (corresponding WO 97/13030 and U.S. Pat. No. 5,792,320). This FI patent discloses a method for removing water from a paper or board web and for passing it as a closed draw from a forming wire or transfer wire of a web former section to a press section. In the method of the FI patent, the web running on the forming wire or on the transfer wire is caused to adhere in a transfer and pre-press zone to the outer face of a transfer belt which is substantially non-water-receiving, and after the pre-press zone, the web is separated substantially immediately from said wire and passed on support of the transfer belt loop onto the next press fabric and/or into the next press nip in the press section.
In addition, the above-mentioned FH patent discloses a press section in which the press section includes a pre-press zone or zones and a transfer belt loop which is substantially non-water-receiving and has an outer face capable of adhesion to the paper web. This transfer belt loop is passed through the pre-press zone, or if two zones are present, at least through the latter zone. In the pre-press zone, the paper web is caused to adhere to the outer face of the transfer belt loop, and after the zone, it is separated substantially immediately from the forming wire without substantial rewetting of the web. On the transfer belt, the web is passed as a closed and supported draw onto the next press fabric and/or through the next press zone in the press section.
In FI patent 98843, a reliable and closed draw of the web is accomplished from the former section to the dryer section without risk of rewetting of the web. Also, in connection with the forming wire or an equivalent transfer wire it is possible to arrange one or more pre-press zones in which the web is caused to adhere reliably to the transfer belt which is substantially non-water-receiving, and moreover, a substantial amount of water is removed, which increases both the dry solids content and the wet strength of the web. This again improves the runnability of the press section and facilitates later stages of dewatering.
Drawbacks of the method and the press section disclosed in the above-mentioned FI patent 98843 include at especially high web speeds in particular difficulties in passing the paper web and the edge strips cut therefrom into a pulper in connection with threading or a disturbance in operation, since in the FI patent, the upper fabric tends to carry the web with it, although the pre-press nip would be even open. This causes web breaks and other operational malfunctions in the press section.
With increasing speeds of paper machines, the above-noted web transfer, pick-up and dewatering capacity problems and other difficulties associated with them are aggravated.
An object of the present invention is to develop further the method for transferring and pressing a paper web and the press section disclosed in the above-mentioned FI patent 98843 so that the advantages attainable by the FI patent are also retained in the present invention.
An object of the present invention is to provide a new combination of a former and a press section of a paper machine as well as a method in the transfer of a web from the former section to the press section so that the above-noted drawbacks are mainly avoided and the other objects of the invention explained later are achieved.
With a view to achieving the objects stated above and those which will come out later, the former-press section in accordance with the invention is mainly characterized in that said upper wet wire or an equivalent transfer wire of the former section is passed through said pre-press zone, the web being passed on the lower face of said wire into and through said pre-press zone, that inside the loop of said upper wet wire or of said equivalent transfer wire and/or inside the loop of a lower wet wire there are transfer devices based on the use of a pressure difference, by means of which transfer devices it is ensured in a normal situation of operation that the web is separated from the lower wet wire and follows the upper wet wire or the equivalent transfer wire and runs on the lower face of said wire further into the pre-press zone, and that said transfer devices based on the use of a pressure difference can be arranged to operate in connection with threading or an operational malfunction such that the web is separated from the upper wet wire or the equivalent transfer wire and falls down into a wire pit, pulper or equivalent situated underneath.
The method in accordance with the invention is in turn mainly characterized in that, after forming of the web or at its final stage, a pressure difference is applied to the web through said wet wires and through a transfer wire, if any, by means of particular pressure difference devices such that the pressure acting on the web is arranged in said pressure difference devices to be higher on the side of an upper wet wire or an equivalent transfer wire with the result that, in connection with threading or an operational malfunction, the web is passed to follow a lower wet wire, and said pressure is arranged to be higher on the side of the lower wet wire with the result that, in a normal situation of operation, the web is passed to follow the upper wet wire or an equivalent transfer wire.
By means of the method and the device of the invention the web is made to reliably follow the upper wet wire or an equivalent transfer wire in a normal situation of operation directly into a pre-press zone or, alternatively, in case of a disturbance in operation or in connection with threading, the web is made to reliably follow the lower wet wire before the pre-press zone so that the web and its edge strips do not cause disturbance in the pre-press zone and in the press zones proper following after it. The devices based on a difference in pressure in accordance with the invention enable said transfer to be efficiently ensured because the permeability of the wet wire or wires and the transfer wire, if any, is relatively high and thus, through it, the effect of a pressure difference can efficiently be applied to the web without excessive energy consumption or other disturbing factors. In the pre-press zone applied in the invention, because of the above-mentioned high permeability of the wires, considerable amounts of water can be removed using a moderate amount of energy so that the web can be passed even at high speed in a reliable manner to the press section proper and through it and further to a dryer section. Moreover, the formation of the web and its z-direction distributions can still be affected efficiently in the pre-press zone.